In a view point of an energy problem and an environmental problem, attention is given to an artificial photosynthesis technology which artificially produces a storable chemical energy source from solar energy by imitating photosynthesis of plants. In the artificial photosynthesis technology, it is studied to use a photo-electrochemical cell which has a photoelectric conversion layer using a semiconductor, an oxidation catalyst layer, and a reduction catalyst layer. According the photo-electrochemical cell, a high efficiency is obtained by using a semiconductor layer excellent in light absorption and charge separation. A photo-electrochemical cell of a monolithic structure in which an oxidation catalyst layer and a reduction catalyst layer are disposed on both sides of a photoelectric conversion layer, since having a structure that does not require wiring, has characteristics that efficiency decrease due to wiring resistance does not occur, that size increase is easy, and that a manufacturing cost is low.
Since a photo-electrochemical cell which has a photoelectric conversion layer using a semiconductor is used while being immersed in an electrolytic solution, there is a possibility that a semiconductor layer is corroded by the electrolytic solution. For example, a catalyst layer formed on a light receiving surface side of the photoelectric conversion layer is formed into an island shape or a thin film shape in consideration of a light transmission property. In a photo-electrochemical cell having an island-shaped catalyst layer, though an incident light amount to a photoelectric conversion layer is large, a durability is decreased due to an existence of an exposed surface. In a photo-electrochemical cell having a thin film-shaped catalyst layer, a durability is high since an exposed surface does not exist, but on the other hand, an efficiency is decreased due to occurrence of an optical loss in the catalyst layer. In a conventional cell structure, an efficiency and a durability are in a relation of trade-off.
With regard to a conventional photo-electrochemical cell, as a measure to improve a durability (corrosion resistance), it is studied to prevent direct contact of a semiconductor layer to an electrolytic solution by forming a transparent conductive film which has corrosion resistance on a photoelectric conversion layer, or by forming a photocatalyst layer made of a titanium oxide, a tungsten oxide, or the like. However, in the former measure to improve a durability, since the catalyst layer is provided in a side surface portion of the cell, an electron and a positive hole generated in the photoelectric conversion layer receive a resistance loss of the transparent conductive film. Therefore, decrease in efficiency is incurred as a cell size becomes larger. In the latter measure to improve a durability, because of an equivalent circuit in which the photoelectric conversion layer and the photocatalyst layer are connected in series, an efficiency which is able to be obtained is limited by a capability of a photocatalyst.